The present invention relates to a wireless communication system and abase station, and more specifically, to a wireless communication system that urges a handover processing of a mobile terminal of interference source in a heterogeneous network where a base station whose output is large and a base station whose output is small exist together.
As a background art of this technical field, there is LTE (Long Term Evolution) that is a cellular system to which OFDMA (Orthogonal Frequency Division Multiple Access)-MIMO (Multiple Input Multiple Output) technology is applied, which has already come to a stage of service-in in some countries.
In the cellular system in recent years, not only a large coverage and large output macro base station that was formerly a mainstream but also smaller coverage and smaller output pico base station and femto base station increase in number, which are used for a use of covering a coverage hole of the macro base station and a use of shifting a terminal connecting to the macro base station to a small coverage base station apparatus side by placing it in an area where the data traffic is locally high, called a hot spot, namely a use for traffic offloading.
A network where the large coverage base station and the small coverage base station exist together is termed a heterogeneous network. As a problem peculiar to the heterogeneous network, a downlink cell edge and an uplink cell edge may not be in coincidence with each other. Generally, regarding the cell edges formed between the large coverage base station and the small coverage base station, the uplink cell edge is located near the large coverage base station compared to the downlink cell edge. In the LTE, since the terminal connects to the base station such that a downlink received power therefrom is high, the terminal in the downlink cell edge connecting to the large coverage base station will transmit an uplink signal at a position nearer the small coverage base station than that in the uplink cell edge. As a result, it will transmit an uplink interference signal whose received power is large to the small coverage base station located in the neighborhood of the terminal.
Japanese Unexamined Patent Application Publication No. 2009-303223 discloses one of methods for solving this problem. This patent literature presents, in summary, that the small coverage base station detects the terminal connecting to the large coverage base station that may exert an interference of high power to the small coverage base station, and notifies detected information to the large coverage base station to urge it to perform handover to the small coverage base station.
A handover related processing in the LTE is disclosed, for example, in (1) 3GPP, “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 (Release 10),” TS36.300, pp. 60-91, v 10.3.0, 2011/3, (2) 3GPP, “Evolved Universal Terrestrial Radio Access Network (E-UTRAN); S1 Application Protocol (S1AP) (Release 9),” TS36.413, pp. 75-82, v 9.1.0, 2009/12, (3) 3GPP, “Evolved Universal Terrestrial Radio Access Network (E-UTRAN); X2 Application Protocol (X2AP) (Release 9),” TS36.423, pp. 12-17 and 27-32, v 9.1.0, 2009/12, (4) 3GPP, “Evolved Universal Terrestrial Radio Access (E-UTRA); Radio resource Control (RRC); Protocol Specification (Release 10),” TS36.331, pp. 47-49, pp. 119-120, pp. 191-192, v 10.0.0, 2010/12, and (4) 3GPP, “Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control(MAC) protocol specification (Release 10),” TS36.321, pp. 12-17, v 10.0.0, 2010/12. An outline of a handover method described in 3GPP, “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 (Release 10),” TS36.300, pp. 60-91, v 10.3.0, 2011/3 will be explained. In the explanation, it is premised that the terminal is connecting to the large coverage base station, the small coverage base station exists in the neighborhood of the terminal, and the terminal is to be moved to the small coverage base station from the large coverage base station.
As preliminary preparation before performing the handover, the large coverage base station configure the terminal to measure the downlink received power about the large coverage base station and the neighbor base stations, i.e., the small coverage base stations in the case explained here, and report it. The terminal feeds back a measurement result of the downlink received power to the large coverage base station according to a configuration result. The large coverage base station refers to feedback information coming from the terminal, and if the downlink received power that is the downlink received power about the small coverage base station added with the a predetermined bias becomes larger than the downlink received power about the neighbor large coverage base station, it will start the handover processing of the terminal to the small coverage base station.
In the handover processing, first, the large coverage base station issues a Handover Request to the small coverage base station that becomes a destination of the terminal. If an ACK comes back from the small coverage base station, a Handover command will be transmitted to the terminal so that the terminal shall move to the small coverage base station. Upon reception of the Handover command, the terminal performs downlink reception timing synchronization and an initial access operation to the small coverage base station described in the Handover command. Subsequently, the small coverage base station gives a terminal ID that is distributed locally within the small coverage base station, and preparation of data communication is completed on the radio side. On the other hand, in order to hand over the data communication of the terminal between the base stations, a Sequence Number given to a data signal is handed over between the base stations. Moreover, since it is necessary to know which base station takes a control of the terminal when seeing it from a network host side, information on the original base station and the destination base station of the terminal is reported to a Mobility Management Entity (MME) from the base stations that are involved in the handover.